The present invention relates to an improved apparatus for controlling an elevator which is driven by an AC motor.
According to conventional apparatus of this type, an induction motor is used for driving a cage of the elevator, and AC power whose voltage and frequency may be varied through an inverter, is supplied to the motor to control the running speed.
FIG. 1 is a diagram of an electric circuit of a conventional apparatus for controlling an AC powered elevator, wherein terminals R, S and T are connected to a three-phase AC power source; a rectifier 1 converts a three-phase AC power source voltage into a DC voltage; a smoothing capacitor 2 is connected to the DC side of the rectifier 1; a main inverter 3 of the widely known pulse width modulation type is connected to the DC side of the rectifier 1 and converts a predetermined DC voltage into an alternating current having a predetermined voltage and a predetermined frequency; a separately-excited inverter 4 for regeneration consists of a thyristor connected to both sides of the rectifier 1 and sends the DC power back to the input side of the AC power source; an auxiliary inverter 5 is connected to the DC side of the rectifier 1 and is constructed in the same manner as the main inverter 3; contacts 6a-6c of an electromagnetic contactor are connected to the AC side of the main inverter 3 and are closed when the operation is to be commenced; contacts 6a-6c are opened when the operation is halted and are also opened in case the main inverter 3 becomes defective and a defect detector (not shown) is activated; contacts 7a-7c of an electromagnetic contactor are connected to the AC side of the auxiliary inverter 5 and are normally open but are closed in case the main inverter 3 becomes defective and the defect detector is activated; a three-phase induction motor 8 is connected to the contacts 6a-6c and to the contacts 7a-7c; a drive sheave 9 of a winch is driven by the motor 8; a main rope 10 is wound on the sheave 9; a cage 11 is coupled to one end of the main rope 10, and a balancing weight 12 is connected to the other end of the rope 10.
The contacts 6a-6c of the electromagnetic contactor are closed when the operation is commenced normally and the contacts 7a-7c are opened. Therefore, the motor 8 receives AC power of a voltage and a frequency that may be varied by the main inverter 3. Therefore, the motor 8 is actuated, and the cage 11 runs. When the cage 11 is descending with a heavy load, or is ascending with a light load, or is decelerating, the mechanical energy is converted into electrical energy and is sent back to the DC side via the main inverter 3. Various defects develop if the returned electrical energy is left untreated. The regeneration inverter 4 operates to treat the regenerated electric power, i.e., the inverter 4 sends the electrical power supplied to its DC side back to the three-phase AC power source at terminals R, S, and T.
The main inverter 3 consists of diodes and transistors, and exhibits complex functions such as switching functions and commutation functions. Therefore, in the control apparatus of FIG. 1, trouble mainly develops in the main inverter 3. If trouble develops in the main inverter 3, contacts 6a-6c are opened, contacts 7a-7c are closed, the motor 8 is driven through the auxiliary inverter 5, and the cage 11 stops at the nearest floor.
Since the auxiliary inverter 5 is employed, to maintain safety even in the case where the main inverter 3 becomes defective, the control apparatus becomes complex in construction and becomes expensive.